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[SR-8077] Compile error when using @dynamicMemberLookup as a protocol requirement #50610

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ole opened this issue Jun 22, 2018 · 6 comments
Closed

[SR-8077] Compile error when using @dynamicMemberLookup as a protocol requirement #50610

ole opened this issue Jun 22, 2018 · 6 comments
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@dan-zheng
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself

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@ole
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ole commented Jun 22, 2018

Previous ID SR-8077
Radar None
Original Reporter @ole
Type Bug
Status Resolved
Resolution Done
Environment

Tested in:

  • Xcode 10b2, Apple Swift version 4.2 (swiftlang-1000.0.16.9 clang-1000.10.25.3)

  • and with the Swift-Development-Snapshot-2018-06-20-a toolchain.

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug
Assignee @dan-zheng
Priority Medium

md5: 5a6078ecbd3a511f4ebcc3bee56fffa2

Issue Description:

See the associated Swift forums discussion.

Using @dynamicMemberLookup as a protocol requirement without a default implementation for the subscript produces a compile error:

// error: @dynamicMemberLookup attribute requires 'P'
// to have a 'subscript(dynamicMember:)' member with
// a string index
@dynamicMemberLookup
protocol P {
    subscript(dynamicMember member: String) -> String { get }
}

Adding a default implementation for the subscript compiles and works correctly:

@dynamicMemberLookup
protocol P {
    subscript(dynamicMember member: String) -> String { get }
}

extension P {
    subscript(dynamicMember member: String) -> String {
        return ""
    }
}
@dan-zheng
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This is because subscript(dynamicMember🙂 lookup does not search through protocol requirements, only concrete methods.

This is a bug, not a feature. The @dynamicCallable implementation fixed this bug but the logic needs to be backported to @dynamicMemberLookup. I'll handle this before the Swift 5 release cutoff.

@dan-zheng
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Fixed in #20516
subscript(dynamicMember🙂 methods now work as @dynamicMemberLookup protocol requirements.

@ole
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ole commented Nov 14, 2018

@dan-zheng Awesome, thanks for working on this!

@dan-zheng
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My pleasure!
Now, we can implement "true" @dynamicMemberLookup protocols.

@swift-ci
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Comment by Frank Valbuena (JIRA)

The fix worked however when trying to use it on a conforming type the autocompletion of the properties is broken. Tested on Xcode 11 beta 5.

@swift-ci
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Comment by Frank Valbuena (JIRA)

The previous comment is for SE-0252 Swift 5.1

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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@dan-zheng dan-zheng

Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself
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@ole @dan-zheng @swift-ci